thirties,the numbers, years, degrees, or the like, from 30 through 39, as in referring to numbered streets, indicating the years of a lifetime or of a century, or referring to degrees of temperature: He works in the East Thirties. She must be in her thirties. The temperature was in the thirties yesterday.

early 15c. metathesis of Old English þritig, from þri, þreo "three" + -tig "group of ten" (see -ty (1)). Cf. Old Frisian thritich, Old Saxon thritig, Dutch dertig, Old High German drizzug, German dreissig. Thirty Years' War (1842) was a religious power struggle waged 1618-48, mainly on German soil. The symbol -30- as printer and telegrapher's code to indicate the last sheet or line of copy or a dispatch is recorded from 1895. In 20c. jargon of journalism, it came to be a traditional sign-off signal and slang word for "the end."